1. Field of the Invention
The present invention relates to a method of manufacturing gradient or distributed index optical elements with the use of a sol-gel process applied to the manufacture of optical elements such as an optical lens.
2. Description of the Related Art
In manufacturing gradient index optical elements having a gradient index in the diametrical direction, hitherto, there has been carried out a sol-gel process.
As a method of producing a concentration gradient in the sol-gel process, the methods described in, for example, "Journal of Non-Crystalline Solids", Vol. 100 (1988), pp. 383-387, are well known. In these methods, a sol is prepared by adding a titanium metal in the form of a metal alkoxide as a source of index modifying cations (dopant) to a silicon alkoxide for forming a high refractive index. After subjecting the silica sol to a gelling treatment to form a wet gel, this wet silica gel is dipped in a solution such as an acid capable of eluting the above dopant. After washing any residual eluting solution, the silica gel is dried and sintered.
In the gradient or distributed index optical elements formed in the above-described conventional manufacturing method, the step of imparting a gradient index is performed only one time. In this case, the refractive index at the outer periphery portion of the gradient index optical elements is decreased to some extent, but the concentration of the dopant contributing to formation of the refractive index distribution is not sufficiently decreased, so that a large value of difference in refractive index (.DELTA.n) between the center portion and the outer periphery portion of the optical elements can not be obtained.
Since the eluting solution producing a concentration gradient to the dopant in the wet gel does not present dopant into the gel, then theoretical if the gel is immersed in the eluting solution, as shown in FIG. 1 by a dot-dash line 1, the concentration of the dopant at the outer periphery portion of the optical elements becomes zero. However, as shown in FIG. 1 by a solid line 2, actually, the dopant concentration does not become zero, and the refractive index exhibits a high value at the outer periphery portion, so that a large difference in refractive index .DELTA.n can not be obtained to the extend expected from the theoretical value.